A separation technique using a reverse osmosis membrane is widely used for production of plain water by desalination of brine, seawater, or the like, production of ultrapure water, or the like. However, in accordance with continuance of the process, the permeation flux of the reverse osmosis membrane decreases, whereby the operation pressure rises. In this case, in order to restore the membrane performance, a process of stopping the operation and cleaning the reverse osmosis membrane will be needed.
Conventionally, in the case of performing water processing by using a reverse osmosis membrane, in order to reduce the frequency of such membrane cleaning to enhance the processing efficiency, a method is carried out in which the supply water to the reverse osmosis membrane apparatus is evaluated by various methods, and a pre-processing is carried out in accordance with the needs so that this value will be less than or equal to a predefined value so as to make the reverse osmosis membrane supply water be clean to some extent, whereby obstacles such as decrease in the permeation flux or rise in the operation pressure in the reverse osmosis membrane apparatus are avoided so as to continue a safe operation.
For such an evaluation, there is known a fouling index (FI) defined in JIS K3802 or a silt density index (SDI) defined in ASTM D4189 or, as a more convenient evaluation method, an MF value proposed by Taniguchi (Desalination, vol. 20, pp. 353-364, 1977).
These FI value, SDI value, and MF value are all calculated based on a measurement value obtained by measuring a predetermined filtration time when reverse osmosis membrane supply water is filtered with use of a precision filtration membrane having a pore diameter of 0.45 μm. In the following patent document 1, a reverse osmosis membrane apparatus is disclosed in which operation management is carried out based on an evaluation result obtained by evaluating whether the water supplied to the reverse osmosis membrane apparatus is good or not with use of such a precision filtration membrane.
However, with this reverse osmosis membrane apparatus, through operation management is carried out by evaluation of the supply water, when a problem such as scale is actually generated in the reverse osmosis membrane by some cause, this cannot be evaluated by the precision filtration membrane of the supply water.
Also, the following patent document 2 discloses a membrane separation apparatus in which a submodule is provided in parallel with a main module, and an operation state of the submodule is monitored while performing reverse osmosis membrane separation under the same condition by feeding the same supply water. Then, it discloses that, as a submodule, the membrane surface has a small area; the permeation flux is monitored; and the effective difference pressure is monitored.
However, when a problem such as scale is generated, for example, in this membrane separation apparatus, in order to detect this with certainty, a devise in the structure will be needed such as providing a structure in which the concentration polarization in the supply-side flow path is less liable to be generated in the submodule than in the main module, thereby raising a problem in that the certainty and specificity of detection are low.    Patent document 1: Japanese Unexamined Patent Publication No. 2004-188387    Patent document 2; Japanese Unexamined Patent Publication No. 10-286445